1. Field of the Invention
The present invention is related to the image processing of pixelised images, more particularly to the judgement whether a pixelised image is a monochrome or a color image. The image processing system may be a stand-alone system or may be part of a scanning and/or image recording and/or printing and/or copying system.
2. Discussion of the Related Art
A pixelised image is a digital image or document composed of pixels (picture elements). There are numerous ways to generate a pixelised image such as for instance, by scanning an original image or document using a scanner, or by generating still images using a digital camera or a video camera. Besides the pixelised images generated by a scanner or a digital camera, which are usually in a bitmap format or a compressed bitmap format, also artificially created, e.g. by a computer program, digital images or documents may be offered to the image processing system. The latter images are usually in a structured format including but not limited to a page description language (PDL) format and an extensible markup language (XML) format. Examples of a PDL format are PDF (Adobe), PostScript (Adobe), and PCL (Hewlett-Packard). These structured images can easily be converted by a raster image processor into pixelised images.
The pixelised images may comprise color data including color image data, color errors, e.g., introduced by scanning a black and white original with a color scanner, or color background data, e.g., when scanning an original image printed on a colored background with a color scanner.
For each pixelised image, or parts thereof, the image processing system has to judge whether it is a monochrome image or a color image. The reliability and quality of this judgement is important for many reasons related to the subsequent processing of the image. One of these reasons is for instance image processing time and memory allocation, particularly when storing the pixelised image. When a monochrome image is misjudged as a color image, the processing time and storage capacity required is considerably larger compared to storing the image when being judged as a monochrome image. Another reason can be image quality when reproducing the image. For example when a monochrome original is scanned by a color scanner, recognition of the pixelised image as a color image and subsequent reproduction thereof by a color image forming system could result in a severe image quality degradation due to, e.g., registering errors. Systems used to reproduce pixelised images include display systems, scanning systems and printing and/or copying systems. A further reason can be throughput and cost as image reproduction on a monochrome image reproduction system is usually cheaper and faster compared to reproduction on a color reproduction system. Particularly in a professional environment such as e.g. the repro department of a company or a print and/or copy shop, where multiple image reproduction systems are available, including a color image reproduction system and a black and white image reproduction system and/or a color image reproduction system capable of operating in a “black only” mode, it is desirable to make optimal use of the resources in order to optimize throughput and costs. Also for the reproduction of a mixed color image, being an image or a document comprising both colored and non-colored parts, it may be beneficial to split the mixed color image into a color part and a monochrome part.
Image processing systems are known, as for instance disclosed in European Patent Application Publication No. EP363146. Such systems are able to split mixed color images on page level into a color part and a monochrome part. The color part contains all the pages of the mixed color image with color information. The monochrome part contains all the pages of the mixed color image with black and white information only. Subsequently, the respective parts are processed by a color image reproduction system and a black and white image reproduction system respectively. The sole criterion to judge whether a page is to be classified as a color page or not is the bare presence of some color information.
More advanced algorithms could be used to provide the operator and/or user with the possibility to define a threshold in the color content, i.e., the amount of color information present on an image page, or image part or image. If the amount of color is below the color threshold, the image is subsequently treated as a monochrome image, even although it may contain a limited amount of color information, e.g., a colored line or bullet.
An image processing system disclosed in European Patent Application Publication No. EP 0 448 330 firstly converts the image data of the pixelised image to a CIE color space. EP 0 448 330 contains embodiments wherein the saturation value of each individual pixel is compared to a first threshold in order to judge whether it is a color pixel or not. The color pixels are counted and compared with a second threshold in order to judge whether the image is a color image or not. Although this solution may definitely have some merit, a disadvantage thereof is that the color judgement is performed on individual pixels and is inherently sensitive to color errors, e.g., pixels having a saturation value different from the original due to a scan read and/or calibration error. EP 0 448 330 further discloses to perform the color judgement in accordance with a frequency distribution of saturation information of each pixel. In particular, pixels having a saturation value with a high occurrence in the image are decisive in the color judgement. But a disadvantage of this disclosed method is its lack of sensitivity when judging images having a limited amount of color, in other words images comprising only color pixels having a saturation value with a low occurrence in the image.